Devices that reduce exposure to residential airborne contaminants, such as allergens and pollutants, are useful in residential and institutional settings. Clean air technology is highly effective at removing airborne particles by passing an ambient air stream through High Efficiency Particulate Air (HEPA) filters. However, the efficiency of HEPA filtration systems depends on airflow dynamics of the environment in which the device is used. In-mixing of contaminated ambient air with filtered air typically diminishes the ultimate efficiency of HEPA filtration.
Room air cleaner units thus cannot typically displace body convection and provide a controlled personal breathing zone.
Several devices have been reported that provide a purified personal breathing zone.
WO2008/058538, U.S. Pat. No. 6,910,961, and US2008/0308106 describe specialized air supply outlets that can be positioned to provide conditioned air for a personal clean-air environment.
US2008/0307970 describes a neck-worn device.
U.S. Pat. No. 6,916,238 describes an enclosed clean air canopy that provides a purified personal breathing zone during sleeping hours.
U.S. Pat. No. 7,037,188 describes a bed ventilation system that provides a purified personal breathing zone during sleeping hours.
All of these devices utilize impulse or forced-blown to induce and maintain a stream of filtered air, enveloping a point of care. These methods and devices are however associated with uncomfortable air flow drafts, dehydration and an overall poor control of the filtered air stream velocity. Further, even where the filtered air stream is substantially laminar, the sometimes high velocities of forced-blown air inevitably invoke turbulent in-mixing of contaminated ambient air, in the absence of a canopy or enclosure.
Turbulent in-mixing of ambient air can be avoided by utilizing gravity to induce a laminar air flow, rather than impulse or blowing force. The principle of TLA is that a laminar flow is induced by an air-temperature difference between supply air and ambient air at the point of care. A substantially laminar flow of filtered, colder air, having a higher density than ambient air descends slowly, enveloping the breathing zone of a sleeping person. The TLA principle provides an unprecedented ability to control the air flow velocity as measured at the point of care. Parts of or the whole temperature control device may be situated before or after the blower device supplying the laminar air flow. Temperature controlled laminar air flow (TLA) is based upon boundary control and unidirectional orientation of a laminar air supply structure. Stable flow conditions are maintained by introducing a temperature gradient (negative buoyancy) between the cooled supply air and ambient air in the human breathing zone. Entrainment including turbulent diffusion of ambient air into the laminar supply stream is here limited to a minimum. The filtrated and cooled laminar air, with higher density than ambient air, descends slowly enveloping the breathing zone of a person in bed. Because the air flow is substantially laminar, and entrainment of ambient air is avoided, the air-temperature difference is maintained throughout the path of descent. This downward directed displacement flow will unaffected pass physical obstacles in the air-flow path. A free and isothermal jet flow loses momentum after bouncing off physical obstacles. In contrast, the cooled TLA air retains its lower temperature despite interactions with physical obstacles. TLA thus provides improved removal of contaminants from the breathing zone to the floor level.
To be effective in providing a controlled personal breathing zone, a TLA device will ideally provide a substantially laminar descending air flow having sufficient velocity to displace convection currents caused by body heat. A warm human body causes a convection air flow having an ascending velocity of over 0.1 m/s and having an air-temperature increased as much as 2° C. above ambient air at body level. An effective TLA device thus typically provide a descending, substantially laminar flow of filtered air with velocity >0.10 m/s, and in any case, sufficient to break body convection currents.
Excess velocity of filtered air is, however, undesirable. Excess air flow velocity gives rise to drafts, which are both uncomfortable and, also, dehydrating. Avoiding drafts and dehydration is pivotal for the long term compliance by patients/users. Bare parts of the human body are extremely sensitive for air movements during low activity or sleep. Furthermore, the greater the velocity of the descending laminar air stream, the more difficult it is to control and direct it to the point of care without in-mixing of ambient air.
In a TLA device, the velocity of the descending air stream is determined by the air-temperature difference (i.e. density differences) between the colder, filtered supply air and the ambient air at the level of the point of care. Only minimal impulse is imparted to the air stream, sufficient to overcome resistance at the outlet nozzle.
U.S. Pat. No. 6,702,662 describes a device that utilizes TLA to provide a personal breathing zone. In this device, filtered air is divided into two partial air streams one of which is cooled, the other heated. The cooled air descends to a breathing zone from a laminar flow air supply nozzle. The heated partial air stream provides a controlled thermal stratification of the room, ensuring that the cooled air stream will descend free of interference from the uprising heated air stream. This device provides filtered air simultaneously to a personal breathing zone and to an entire room.
The requirement for two filtered air streams gives rise to several disadvantages. First, the device is physically more bulky than a device having only a single filtered air stream. Second, a greater volume of air flow is required for two air streams, which is associated with an increased requirement for fan or blower activity. Noise generated by a fan or blower is undesirable in a personal breathing device suited for use with sleeping patients. Third, use of this device can give rise to unwanted drafts. Because the cool partial air stream can only be cooled, the device is unable to accommodate circumstances which can arise in home use where a pre-existing air-temperature gradient exists within a room. In some circumstances, air taken in at floor level can already be significantly cooler than air at the level of the personal breathing zone. In the absence of some capacity for heating the supply air stream, an excessive descending velocity of filtered air can result, causing drafts.
In clinical trials using one embodiment of the TLA device described in U.S. Pat. No. 6,702,662, we discovered that a relatively narrow range of conditions exists in which it is possible to avoid drafts (caused by excessive velocity of the descending air stream) while also avoiding inability to displace warm body convection currents (caused by insufficient velocity of the clean air stream). We have determined that an optimum air-temperature difference between the filtered, descending laminar air and the ambient air at the level of the personal breathing zone falls within a range of about 0.3 to 1.0° C.
This optimum range can be provided by methods and devices of the present invention, which do not require two partial streams of filtered air. Only a single filtered air stream is subject to temperature adjustment. In preferred embodiments, air-temperature of the filtered air can be carefully adjusted via a temperature control system to maintain, within the optimum range, an air-temperature difference between supply air and ambient air at the level of a personal breathing zone. Reversible polarity of the thermoelectric cooler (TEC) used to provide air-temperature adjustment permits the supply air stream to be alternately cooled or heated, thereby providing necessary fine tuned control of descending air stream velocity.
By avoiding a heated secondary air flow, a TLA device can be provided that is smaller in size and thus better suited for comfortable home use. Comfort for sleeping users can also be increased by reduced fan noise.